The present invention relates to regulated power supply circuits and regulated power supply devices, and in particular, overcurrent protection mechanisms.
We will first discuss conventional regulated power supply circuits in reference to FIG. 11 which is a circuit block diagram representing a dropper-type regulated power supply circuit 51. In the regulated power supply circuit 51, a PNP power transistor TR1 hereinafter, transistor TR1 acting as an output control element drops an input voltage Vin to produce an output voltage Vo. During normal operation, an error amplifier 53 compares a partial voltage of the output voltage Vo detected by resistors R1 and R2 to a reference voltage supplied from a constant voltage circuit 54. The error amplifier 53 adjusts the base current of a transistor TR5 according to an error obtained from the comparison, which in turn adjusts the base current of a transistor TR1 via a transistor TR2 to keep the output voltage Vo at a constant value.
However, an overcurrent flowing through the collector of the transistor TR1 reduces the output voltage Vo. In contrast, under a normal condition, an overcurrent detection circuit 52, upon detection of such an overcurrent, directly varies the base potential of the transistor TR1 for the purpose of restraining the current flow.
When a load R3 is short-circuited, the output voltage V0 of the transistor TR1 falls to GND level, and so does the potential at the contact between the resistors R1 and R2. Therefore, in the short circuit protection circuit 56, the base potential of the transistor TR3 falls. This turns on the transistor TR3 and thus produces a current flow to the resistor R4, which then elevates the base potential of, and thus turns on, the transistor TR4. As a result, the current flow from the constant current circuit 55 to the base of the transistor TR5 branches off to be coupled to the collector of the transistor TR4. This reduces the base current of the transistor TR5 and causes the current flow to the base of the transistor TR2 to branch off to the emitter, then the base, of the transistor TR3, and further pass through the diode D1 and the resistor R2. Accompanying great fall in the base current of the transistor TR2 causes a great fall in the base current of the transistor TR1, limiting the output current Io of the transistor TR1.
When the output short circuit is incomplete as described above, however, a problem develops such that great loss occurs at the transistor TR1. To explain this phenomenon, reference is made to FIG. 12 showing overcurrent limiting characteristics representing a relationship between the output voltage Vo and output current Io of a typical regulated power supply circuit.
A complete short circuit, denoted by point A in FIG. 12, is a condition in which a resistor with a small impedance is used to short-circuit the output terminals to completely short-circuit the output. At point A, a loss of (Vinxe2x88x92Vo)xc3x97Io occurs in the transistor TR1; however, the output current Io is limited to a small value, and the loss is relatively small.
In contrast, an incomplete short circuit, denoted by point B in FIG. 12, is a condition in which a resistor with a greater impedance than in the case of a complete short circuit is used to short-circuit the output terminals. At point B, the output current Io is not limited, and the loss of (Vinxe2x88x92Vo)xc3x97Io grows large. If the device is in such a condition for an extended period, various inconveniences will entail including breakdown of the device and heating of the printed wire board.
Although the foregoing description clearly predicts the importance of curbing the loss in the output control element when the short circuit is incomplete and an overcurrent is detected, attempt to be compatible with every kind of overcurrent results in restraining the indispensable overcurrent that occurs inevitably during a startup in which the output voltage Vo rises to an operating level. This renders it to difficult to start up the device.
The present invention has an objective to provide regulated power supply circuits and regulated power supply devices such that no loss develops in the output control element when overcurrent flows, except during a startup.
A regulated power supply circuit in accordance with the present invention, in order to achieve the above objective, includes:
an output control element;
an overcurrent detection circuit for detecting an overcurrent flowing through the output control element;
a cut-off circuit for cutting off an output of the output control element when the overcurrent detection circuit detects the overcurrent; and
a cut-off operation prohibition circuit for prohibiting the cut-off circuit from performing the cut-off operation from an outset to an end of a startup of the regulated power supply circuit.
According to the invention, when the overcurrent detection circuit detects an overcurrent flowing through the output control element, the cut-off circuit operates to cut off the output of the output control element. This way, if an overcurrent is caused by a short circuit, whether complete or incomplete, no current is allowed to run through the output control element, thus developing no loss in the output control element. However, the output voltage will not build up provided that the output of the output control element is cut off in response to an overcurrent that occurs inevitably during a startup. Therefore, the cut-off operation prohibition circuit is adapted to prohibit the cut-off circuit from performing the cut-off operation from the outset to the end of the startup.
Hence, a regulated power supply circuit can be provided which, even if an overcurrent is detected, develops no loss in the output control element during non-startup time.
A regulated power supply device in accordance with the present invention, fabricated by sealing an integrated regulated power supply circuit in a single package, in order to achieve the above objective, is such that the regulated power supply circuit includes:
an output control element;
an overcurrent detection circuit for detecting an overcurrent flowing through the output control element;
a cut-off circuit for cutting off an output of the output control element when the overcurrent detection circuit detects the overcurrent; and
a cut-off operation prohibition circuit for prohibiting the cut-off circuit from performing the cut-off operation from an outset to an end of a startup of the regulated power supply circuit.
According to the invention, the regulated power supply circuit is integrated and sealed in a single package and is therefore can be readily mounted on a printed board.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.